Misfire Detection. What sensors involved?

[ksoggs]

Anyone know what sensors are involved in detecting cylinder misfires in the LNF. I always assumed it was the O2 sensors, I was under the impression that if they caught an extremely rich spot in the exhaust stream, that it was assumed that the cylinder must not have ignited, causing the surge of extreme rich in the exhaust. So the ECU sees this spike, and sees where it happens and calculates which cyl should have been exhausting at that time, and throws a misfire code on that cylinder.

I am wondering if i am wrong... Because recently my car has been misfiring all over the place under no-load (according to the ecu). It is perfectly fine under load.. but under deceleration, coasting, or idle... It flashes my check engine light at me, and throws codes for cylinders 1,2,3 and random cyl misfire. (never 0 or 4, always 1 2 3)

At first i noticed most of the time it would scream about misfires, i would see a dip in the readings of the downstream O2 sensor... it would dip almost off the gauge for a second, and then the engine light would start flashing.

BUT i never felt anything... on a 4cylinder at idle/coast/decel you should notice if 1 cylinder chokes... let alone 3/4 of your engine.... i never noticed a thing. was running and idling fine. Even the upstream O2 (wideband) kept a steady reading... Nothing else i can see indicates any sort of misfire, apart from the dip in the downstream O2.....

I do have a hahn catless DP... and i am using the spark plug anti-fowler to remove the O2 from the direct line of the exhaust stream.... I just thought that was the issue... the rear O2 was just acting up from the spacer... (i guess it is common for the spacer to only help for a while then start acting up)

But recently I did the stage one upgrade... and after i flashed the new tune, the misfires all stopped. for about a week... and now they are back again. But this time i dont even notice the dip in the downstream sensor anymore... it is just random misfires with no good reason.

So i am just wondering if anyone knows what could be going on here.... What else might simulate a misfire? shouldnt i feel it sputter or buck or surge or see something in the logs if the engine was actually misfiring? I know my friend had a car with a misfiring cylinder... and that thing would hardly even run with a single cylinder misfire.... the engine was jumping all over, like it was trying to break out of its mounts. I know you get the same reaction if you pull a spark plug wire on most 4cyl... so i really doubt it is a real misfire... but it still bothers me. hoped you guys had some ideas.

[Iam Broke]

The ECU uses an algorythm from the crankshaft sensors to detect a misfire. When I installed a lightweight flywheel on the LNF it got more sensitive to it.

Any abnormal velocity change of the crankshaft is detected as a misfire. Mine would get which cyl it was happening on confused at times when I had a leaking injector seal but it certainly would detect them.

Check your plugs. It probably is misfiring. If they are older than 10 to 15k miles, replace them and gap them to .030 - .032"

Low compression (<90 psi) will cause misfires also, if you've been flogging it a compression and/or leakdown test is in order. Hard to diagnose over the interwebz. If you are having other issues, check the connections & splices on the GMS1 sensor pigtails. They need to be soldered, not crimp spliced.

[ksoggs]

Well the GMS1 kit is better than factory connected. I cut the original connectors off and crimped and soldered the new bosch connectors on. I am certain connections are not to blame here. (mainly because the misfires occured well before i even touched the wiring)

Hmm.. that is dis-heartening to hear that it is actual crank sensors... I was hoping it was all O2 related... and i could blame it all on my crappy spacer for the rear O2 sensor.

the whole car has just over 30k on it... i thought stock plugs were good for 100k+ (arent these non-gappable iridium or something?)

I would hope it isnt compression.... but could low boost levels trigger a misfire? (i turn my DAL's way down in the cruise/coast areas, but never thought this would be a cause for misfires...)

The ECU uses an algorythm from the crankshaft sensors to detect a misfire. When I installed a lightweight flywheel on the LNF it got more sensitive to it.

Any abnormal velocity change of the crankshaft is detected as a misfire. Mine would get which cyl it was happening on confused at times when I had a leaking injector seal but it certainly would detect them.

Check your plugs. It probably is misfiring. If they are older than 10 to 15k miles, replace them and gap them to .030 - .032"

Low compression (<90 psi) will cause misfires also, if you've been flogging it a compression and/or leakdown test is in order. Hard to diagnose over the interwebz. If you are having other issues, check the connections & splices on the GMS1 sensor pigtails. They need to be soldered, not crimp spliced.

[Iam Broke]

Replace the plugs, 20k on them at 20 psi boost is pushing it. OEM plugs are NGK 5476 iirc and one step colder if they look to hot (I doubt it) are NGK 3787's.

I exploded a #2 stock Delco plug at 24 psi with a bunch of timing and no KR logged., paid a few dollars more for the NGK brand even though the Delco's are made by NGK. Mine liked the 3787's but some don't.

If you have 30k on the plugs, I'll bet that's your issue. I started logging more misfires after ~15k.

[Dayhoff35]

mine didnt like the one step colders, or atleast my logs said that it didnt. Thats was only after like 2,500 miles too, i guess i wasted my money there. Im running some iridiums now and everythign seems good.

[Leafy]

You know, hp tuners would let you have that rear O2 show ready all the time and pass plug in emissions without that defoulers, or any rear O2 even hooked up... But thats illegal and we will not endorse that on this site. That function is for off road use only.